Solar collectors receiver tubes

ABSTRACT

Improved solar collectors receiver tubes are described. The improved solar collectors receiver tubes have a getter system containing one or more getter materials in the form of pills housed a single line in a suitable holder.

The present invention refers to improvements for solar collector receiver tubes.

Solar collectors are becoming an alternative energy source of increasing relevance. In these devices, and more specifically in the receiver tubes of solar collectors, the presence of hydrogen is noxious since it increases the heat conduction from the central tubular body, where a heat removal fluid flows, toward the outside of the receiver tube, therefore progressively decreasing its efficiency. In this case the problems linked to the presence of hydrogen are of particular relevance since the fluid flowing in the central body tends to decompose at high temperatures generating hydrogen.

Also in the receiver tubes of the new generations, that employ a different fluid type for the heat removal at higher temperatures, the problem related to the presence of hydrogen and the consequential decay in the device characteristics is of particular relevance since at such higher temperatures there is a higher hydrogen outgassing from the metallic parts of the receiver.

Within the receiver tubes there is also an additional problem related to the high internal temperatures, that bring the getter material into unfavorable working conditions with reference to the hydrogen capacity, that is inversely proportional to the operating temperature.

For this reason some technical expedients have been developed, such as the one disclosed in U.S. Pat. No. 6,832,608, that describes an arrangement, in the form of a sledge, in order to position and efficiently house the getter material within the receiver tube, with the purpose of shielding the getter material from the solar radiation and the portions of the collector that reach the highest temperature.

This type of arrangement has some drawbacks, since typically the getter material is employed in the form of pills of compressed powders having a standardsize, i.e. with a 10 mm diameter, this implies that in order to be able to introduce into the receiver tube the sufficient amount of getter material, to ensure proper device operation for 20-30 years, every sledge shall house two lines of pills, and typically in each receiver tube two sledges are accommodated.

This causes two problems of different nature, on one side a greater difficulty in the loading of the getter material, in the form of two lines of getter pills adjacent to each other, and on the other side in a higher number of contact points of the pills, that are in contact with each other and with the sledge walls.

These contact points create some preferential brittleness regions with consequent powders generation. The generation and consequent detachment of powders from the pills is a phenomenon to be reduced as much as possible, even when those powders are confined by means of suitable containment metallic meshes. These detached powders create problems not only of aesthetic nature, but also of functional nature (for example degrading the transparency of the surface exposed to radiation and consequently the thermal efficiency of the receiver tube).

The purpose of the present invention is to reduce the intensity of this phenomenon, and in a first aspect thereof it consists in a solar collector receiver tube containing a getter system comprising a mechanical support and pills of compressed powders of a getter material, wherein said mechanical support comprises a holder consisting in a base and two lateral shoulders, characterized in that said getter powder pills have a diameter equal to or higher than 15 mm, a diameter/height ratio comprised between 2 and 5, and they are housed within said holder on said base in a single line. The above referenced holder of the getter system can also be described as a U-shaped sledge and the two terms have an equivalent meaning for the purposes of the present invention.

The present invention will be illustrated in the following with the help of the following figures, wherein:

FIG. 1A shows a side view and FIG. 1B a view from above of a first embodiment of the getter system, to be used within the receiver tubes, according to the present invention,

FIG. 2A shows a side view and FIG. 2B a view from above of an alternate embodiment of the getter system, to be used within the receiver tubes, according to the present invention,

FIG. 3 shows some possible embodiments for elastic elements to be used within the mechanical support,

FIG. 4 shows a view from above of a getter system, to be used within the receiver tubes, incorporating one of the elastic elements depicted in FIG. 3,

FIG. 5 is a cross sectional representation of another embodiment of a getter system, to be used within the receiver tubes.

In the figures the dimensions and the dimensional ratios of the elements, with particular yet non-exclusive reference to the dimensions of the metallic mesh openings, are not correct but have been altered in order to improve the comprehensions of the graphical representation of the invention. Moreover, some elements optionally present, such as, for example, optional heat shields or fixing means for the holder/sledge, have not been depicted since they are not essential for its comprehension.

With the present invention the loading operations of the getter material pills into their housing holder are significantly simplified.

It is to be underlined that the increase in the surface of the getter material pills causes a deterioration of their characteristics with respect to the ones that have a smaller surface. This deterioration is nevertheless compensated by the previously described benefits specifically linked to the technical field, i.e. solar collectors receiver tubes.

In fact the pills surface increase, while keeping the same height, renders them more fragile and consequently more difficult to handle, but, on the other side, the height increase results in a negative effect on the pumping speed and in the difficulties that gaseous impurities encounter in reaching the inner pill regions. Therefore the detrimental effects to be expected in adopting pills with an enlarged diameter would have not prompted a person skilled in the art to adopt such a solution. Also it is to be underlined that the getter pills to be used in the present invention are obtained by a suitable compression of the powders of the getter materials and do not comprise binders, that would otherwise affect the capacity of the material intended as amount of hydrogen adsorbed by each pill. This is a fundamental aspect that affects the pill structure resistance and the associated constraints for its successful employ in the solar collector application.

In view of the above described opposite phenomena, the optimal ratio between the pills diameter and their height is comprised between 2 and 5, and for the very same reasons problems start to manifest for pills with a diameter bigger than 35 mm.

Preferably the pills diameter is comprised between 18 and 25 mm and the ratio between their diameter and their height is comprised between 3 and 4. By operating with a holder loaded with pills having such dimensional characteristics, it is possible to insert into the receiver tube a sufficient amount of getter material, capable to guarantee a device lifetime of 20-30 years, by using only one holder/sledge. On the contrary, the arrangements currently employed envision the use of two distinct holders for each receiver tube, with the associated positioning and fixing problems, in addition to the structural costs that include not only the holder but also the elements that complete the mechanical support, i.e. the confinement metallic mesh, spacers and suitable heat shields.

Also it is preferred that that the holder has a width that is 1-3 mm larger than the pills diameter, this solves a technical problem associated with the pill structure swelling as a consequence of the hydrogen sorption by the getter material, by allowing for some degree of accommodation of the pill within the holder.

With regards to the height of the holder, intended as height of the elements that constitute the lateral shoulders, in an even more preferred embodiment envisioning the use of a metallic net as an upper closing system for the holder, such as element 14 in FIG. 1B, the height of the lateral shoulders is 0.5-1 mm less than the pills height. This assures that the net itself acts also a constraining element for the getter material pills, to limit their reciprocal movement and the possibility that as a consequence of hits they fragment with a consequent increase in the powder generation.

In case part of the upper closing system is rigid, as in the case of element 23 depicted in FIG. 2A, it is preferred for the shoulder height to be 1-3 mm more than the pills height for the very same reason and technical effect above described.

In an even more preferred embodiment the holder contains also one or more elastic elements in direct contact with one ore more getter materials pills.

The main function of these elastic elements is to exert a suitable compression force on the pills, pushing them one toward the other. This action guarantees than even though the getter pills are accommodated in a holder, a sledge in its most useful representation, that is larger than their dimension, the compression exerted by the elastic element prevents the movement of the pills within the holder.

The pressure value exerted is a key parameter and its strictly dependent and specifically linked to the field of application, since an excessive pressure value would help in breaking the pills structure rather than preventing its breaking by restraining its movements within the holder. The determination of such value is rendered more difficult by the fact that it should take into account the pill swelling consequent to hydrogen sorption by the getter material, as well as an increased brittleness of the pill structure as a consequence of its hydrogen sorption. The inventors have found that the compression force exerted by the elastic means shall be comprised between 50N and 150N. This value is considered and measured in a newly installed getter systems, therefore when getter material pills may be considered “fresh”, i.e. have adsorbed a minimal amount of hydrogen.

Moreover, the elastic elements shall be free of sharp corners in their contact region with the getter material pills, in order to avoid to have preferential powder generation spots, so they shall present flat or rounded surfaces in such contact points. Such characteristics will be referred hereinafter as “distributed contact”.

With regards to getter materials, preferred are the ones that exhibit a good hydrogen sorption capacity even when used at high temperatures.

Therefore preferred getter materials useful for the present invention are the ones described in U.S. Pat. No. 3,203,901 (Zr—Al alloys), U.S. Pat. No. 4,306,887 (Zr—Fe alloys), GB patent 2077487 (Zr—V—Fe alloys), U.S. Pat. No. 5,961,750 (Zr—Co—R are earths alloy). For hydrogen sorption, particularly at high temperatures, it is also known the use of yttrium alloys, as described in the international patent applications WO 2007/148362, WO 2007/099575 and WO 2010/105945. The above-mentioned getter alloys are the ones preferably used with the present invention, but any hydrogen getter material may be employed with the inventive concept disclosed. Moreover it is possible to envision the use of pills made with a plurality of different getter materials or to load the holder with different types of pills.

In FIGS. 1A and 1B a first embodiment for the getter system 10 for solar collectors receiver tubes is shown, wherein the pills of getter material 11, 11′, 11″ . . . are aligned on the flat base 12 of the holder and a confinement metallic net 14 is present, fixed to the holder, for example by means of electric welding.

In the arrangement shown in FIG. 1A there is also present an optional element 15 that acts as a stopper, mechanically operated, once the pills have been loaded into the holder. FIG. 1A shows only one of such elements 15 close to one of the holderends, but there could be a second stopping elements 15′, as shown in FIG. 1B, at the secondend.

The metallic net can be fixed on the holder in various ways, for example it may be fixed on the holder lateral shoulders 13, 13′, or wrapped and fixed at the flat base 12, on its outer face. In this second case the preferred fixing arrangement provides for a slight overlapping of the edges of net 14, and afterwards performing a single electrical welding in order to fix the net 14 on the holder.

As depicted in FIGS. 1A and 1B, the metallic net can be arranged not over the entire length of the holder, thus leaving the ends free, so as to avoid interferences with possible means required for anchoring the holder within the receiver tube.

In FIGS. 2A and 2B there is shown an alternative embodiment, in which the getter system 20 comprises getter materials pills 21, 21′, 21″ . . . placed on the base of the holder structure, that is closed by means of a cover 23 that comprises openings in which a metallic mesh 22 is arranged.

Cover 23 is then fixed onto the holder base by suitable anchoring means, such as, for example, dews, not shown.

In FIG. 2A there are also shown some tabs 24 folded in such a way as to act as stoppers for the getter material pills 21. 21′, 21″ . . . placed on the holder base.

This arrangement has the advantage of allowing to manufacture the closed holder structure by means of a semifinished product, the holder cover with its metallic mesh fixed upon, and therefore simplify the assembly operations of the getter system used in the present invention.

Also in this second embodiment, in a preferred realization, the holder is slightly wider with respect to the pills diameter and the lateral shoulders height is slightly lower with respect to the pills height. Also in this case the preferred dimensions envision the usage of a holder with a width 1-3 mm larger with respect to the pills diameter, and a lateral shoulders height 0.5-1 mm lower with respect to the pills height. In this embodiment the pills stoppers 24 also serve the purpose of exerting a slight longitudinal pressure on the pills line, therefore limiting their transversal movement.

As far as the characteristics of the metallic net used in the systems according to the present invention are concerned, such as the ones represented in FIGS. 1A, 1B, 2A and 2B previously described, the net has holes, or more generically apertures, that have a size between 10 and 500 μm. Preferably the metallic net openings are comprised between 20 e 50 μm. In case the openings are not circular or roughly circular, the above size refers to their maximum width.

Examples of possible embodiments for elastic elements to be used within the holder are shown in FIG. 3. Each of the depicted elements, 31, 32, 33, 34, has the part that should be in contact with the getter pill, 311, 312, 321, 322, 331, 332, 341, 342 that is flat or rounded, therefore suitable to realize a distributed contact with the getter pill surface.

FIG. 4 shows a view from above of a holder 40, containing one of such elastic elements 41. In FIG. 4 only the cover element of the holder has been represented with the getter material pills 42, 42′ . . . disposed on. It is to be remarked that generally speaking, with reference to the holders of the getter system of the present invention, the cover and base of the holder indentify the parts of the getter system that within the receiver tube are directed respectively toward the outside (the cover) or toward the inner part of the tube (the base). Cover and base therefore refer to the geometrical positioning not to the way the getter system is assembled, as for example in the embodiment depicted in FIG. 4 the getter pills are placed on the would-be cover.

An alternate embodiment of the getter system envisions the use of a modified perforated cover, that instead of presenting a flat surface presents a series of one or more depressions in the net in correspondence to the getter pills, such depressions also presenting rounded or planar contact surfaces in order to achieve a distributed contact interface between the cover and the getter pills.

A cross-section 50 of this arrangement is schematically represented in FIG. 5, where a pill of compressed getter material 51 is disposed over the flat holder base 52, and is vertically restrained by the depressions 53, 53′ of the net 54 being part of the perforated cover 55.

Such an arrangement has the advantage of restraining also the pills with regards to vertical displacements.

The even more preferred embodiment envisions both the use of the depressed perforated cover and of elastic elements for restraining the pills displacement in all directions, but those may also be singly adopted.

As previously described, thanks to the present invention the phenomena that lead to the pills structure disintegration, and therefore to the powders generation, are minimized. Nevertheless such phenomena are always present and have a degenerative nature over time, as a consequence of the pill embrittlement after the hydrogen sorption, whereby it is preferred to provide the holder with a mesh, or net, with a particularly tightweave for an efficient containment of the powders. 

1. A solar collectors receiver tube containing a getter system comprising: a mechanical support, and pills consisting of compressed powders of a getter material, wherein said mechanical support comprises a holder consisting in a base and two lateral shoulders, and wherein said pills do not comprise any binder, have a diameter equal to or higher than 15 mm, a diameter/height ratio comprised between 2 and 5, and are housed within said holder on said base in a single line.
 2. The solar collectors receiver tube according to claim 1, wherein the diameter of the pills is equal to or less than 35 mm.
 3. The solar collectors receiver tube according to claim 2, wherein said diameter is comprised between 18 mm and 25 mm.
 4. The solar collectors receiver tube according to claim 1, wherein said diameter/height ratio is comprised between 3 and
 4. 5. The solar collectors receiver tube according to claim 1, wherein said holder contains also one or more elastic elements in contact with one or more of said pills and said one or more elastic elements present flat or rounded parts for contacting the pills.
 6. The solar collectors receiver tube according to claim 5, wherein said one or more elastic elements exert a compression force onto said pills comprised between 50N and 150N.
 7. The solar collectors receiver tube according to claim 1, wherein said holder is partially wrapped by a metallic net.
 8. The solar collectors receiver tube according to claim 7, wherein said metallic net has openings with a diameter comprised between 10 μm and 500 μm.
 9. The solar collectors receiver tube according to claim 8, wherein said diameter is comprised between 20 μm and 50 μm.
 10. The solar collectors receiver tube according to one of claim 7, wherein said metallic net is fixed onto lateral shoulders of the holder.
 11. The solar collectors receiver tube according to claim 7, wherein said metallic net is fixed on a holder base, on its external.
 12. The solar collectors receiver tube according to claim 7, wherein said metallic net partly overlaps and is fixed on a holder base, on its external side, in its overlapping region.
 13. The solar collectors receiver tube according to claim 1, wherein said holder has a width greater than a diameter of the pills, and a difference between a holder width and the diameter is comprised between 1 mm and 3 mm.
 14. The solar collectors receiver tube according to claim 1, wherein a height of the two lateral shoulders of the holder is less than a height of the pills, and a difference between the height of the two lateral shoulders and the height of the pills is comprised between 0.5 mm and 1 mm.
 15. The solar collectors receiver tube according to claim 1, wherein said holder is closed by means of a perforated cover having a metallic net in cover openings and a height of the two lateral shoulders is 1 mm-3 mm more than the height of the pills.
 16. The solar collectors receiver tube according to claim 15, wherein said metallic net has openings with a diameter comprised between 10 μm and 500 μm.
 17. The solar collectors receiver tube according to claim 15, wherein said perforated cover presents one or more depressions achieving a distributed contact with the pills. 